Metronidazole Dosing Per Kilogram
For adults, metronidazole is dosed at 500 mg orally three times daily (approximately 7.5 mg/kg/dose for an average 70 kg adult), while pediatric patients require 30-40 mg/kg/day divided every 8 hours with a maximum single dose of 500 mg for most infections. 1
Adult Dosing
Standard Infections
- 7.5 mg/kg/dose (500 mg) orally three times daily for 10 days is the typical regimen for most anaerobic infections 2
- For severe infections requiring IV therapy, 500 mg IV every 8 hours is recommended 2
Clostridium difficile Infection (CDI)
- Metronidazole is no longer first-line therapy for CDI; vancomycin or fidaxomicin are preferred 2, 3
- If metronidazole must be used due to limited access: 500 mg orally three times daily for 10 days for nonsevere CDI only 2
- For fulminant CDI with ileus: 500 mg IV every 8 hours combined with oral or rectal vancomycin 2, 1
Pediatric Dosing
Standard Dosing (Children >1 month)
- 30-40 mg/kg/day divided every 8 hours with a maximum single dose of 500 mg 1
- This translates to approximately 10-13 mg/kg/dose every 8 hours 1
Condition-Specific Pediatric Dosing
Clostridium difficile Infection:
- Non-severe CDI: 7.5 mg/kg/dose three or four times daily (maximum 500 mg/dose) for 10 days 1
- Severe CDI requiring IV: 10 mg/kg/dose three times daily (maximum 500 mg/dose) 1
- Note that CDI dosing is lower than for other anaerobic infections 1
Intra-abdominal Infections:
- 30-40 mg/kg/day divided every 8 hours as part of combination therapy with gram-negative coverage 1
- Always combine with aminoglycosides, carbapenems, or advanced-generation cephalosporins 1
Necrotizing Infections:
- 7.5 mg/kg/dose every 6 hours IV combined with cefotaxime or other broad-spectrum agents 1
Crohn's Disease (Perianal Fistulizing):
- 10-20 mg/kg/day in divided doses 1
Neonatal Dosing
- Postnatal age ≤7 days, weight ≤2000 g: 7.5 mg/kg every 12 hours 1
- Postnatal age ≤7 days, weight >2000 g: 7.5-10 mg/kg every 12 hours 1
- Postnatal age >7 days, weight <1200 g: 7.5-10 mg/kg every 8-12 hours 1
- Postnatal age >7 days, weight >2000 g: 10 mg/kg every 8 hours 1
Dosing Adjustments
Hepatic Impairment
- Severe hepatic dysfunction requires dose reduction due to significantly prolonged half-life (up to 42.4 hours vs. 5.9 hours in normal function) 4, 5
- Patients with obstructive liver disease show the lowest clearances (0.281-1.17 ml/min/kg) and require careful monitoring 5
- Consider reducing dose by 50% or extending dosing intervals in severe hepatic impairment 4
Renal Failure
- No dosage adjustment needed for the parent drug as metronidazole pharmacokinetics are unaffected by renal failure 3, 6, 7
- Metabolites accumulate in renal dysfunction but have not been associated with documented toxicity 6, 7
- Hemodialysis removes substantial amounts of metronidazole; consider supplemental dosing post-dialysis 7
Treatment Duration and Safety
Standard Duration
Critical Safety Warning
- Avoid repeated or prolonged courses beyond 10 days due to risk of cumulative and potentially irreversible neurotoxicity (peripheral neuropathy, seizures, encephalopathy) 2, 3, 1
- Patients receiving therapy >10 days require close monitoring for neurotoxic signs 1
Common Pitfalls to Avoid
- Do not use metronidazole as monotherapy for intra-abdominal infections—always pair with gram-negative/aerobic coverage 1
- Do not select metronidazole as first-line for CDI—prioritize vancomycin (125 mg four times daily) or fidaxomicin (200 mg twice daily) 2
- Do not exceed 500 mg per single dose in pediatric patients regardless of weight-based calculations 1
- Do not continue beyond 10 days without compelling justification due to neurotoxicity risk 3, 1
- For fulminant CDI with ileus, do not rely on oral metronidazole alone—use IV metronidazole combined with oral/rectal vancomycin 2, 1